Foreign priority benefit is claimed under 35 USC xc2xa7119 (a)-(d) or xc2xa7365(b) to Norway patent application number 98.3337, filed Jul. 17, 1998.
Not applicable.
1. Field of the Invention
This application relates to riser tubes in connection with petroleum production in deep waters. More specifically, it relates to devices for stabilizing riser tubes, and devices and methods for installing riser tubes between a construction in the sea surface, and an installation on the seabed.
2. Description of the Related Art
Tension leg platforms (TLP) used in deep waters must have some distance between the riser tubes in order to avoid the riser tubes slamming against each other and to avoid the riser tubes becoming tangled up due to movements under varying vertical tension, and due to movements in horizontal sea currents and waves. This invention may be used in petroleum production for sea depths between about 500 meters and 2500 meters. The ordinary way of securing a sufficient separation between the riser tubes is to increase the tension of the riser tubes. A complementary method is to arrange the riser tubes with greater distance at the TLP and at the connection points at a seabed frame. During a deep water field development under heavy sea current conditions one of the considered alternatives was the application of a relatively light TLP with a riser tube separation of 2.4 meters. However, with the initial riser tube separation, there was a risk of the riser tubes touching each other, and the field operator demanded the riser separation be increased to 6.0 meters. This led to abandoning of the use of the initial light TLP, which had to be replaced by a larger TLP at a price six times more expensive, compared to the light platform.
A method to avoid touching between the riser tubes is to tighten them up by means of frames comprised of guide plates in the upper and lower parts of the riser tube tower. The guide plates comprise apertures or openings with fixed separations in the sideward direction, and arranged to be run through by riser tubes and tension legs or tension wires. In Petroleum Geoservices (PGS)"" patent application PCT/NO/00047, an intermediate third frame or guide plate is arranged between the lower and the upper guide plate. This intermediate third guide plate is rotated about a vertical axis in order to tighten up the riser tubes in the sideward direction in a common rotation movement.
An alternative way of separating the riser tubes from each other is to arrange horizontal guide plates with even distance downward along the vertical tension legs and the vertical riser tubes. The horizontal guide plates may in addition be stabilized by wire stays to anchors, preferably suction anchors. First, the tension legs are installed between the seabed installation and the TLP. The guide plates may be arranged with the riser tubes drawn through the guide plates, and where the guide plates are at the outset in a position closely below the bottom of the TLP. One problem which will occur during the installation of riser tube towers of this kind is to hit with the riser tube when it shall be lowered through each guide plate, and additionally to hit the right holes in the guide plate, and to hit the right connection point at the seabed installation. One ordinarily used method is to apply a Remotely Operated Vehicle (ROV) to guide each single riser tube in the sideward direction at both the guide plates and on the seabed installation. However, it is time-consuming and operatively complicated to apply an ROV at such great water depths. Such ROVs are relatively slow, and the energy requirements for supplying energy from the surface to the cable connected ROV is reduced by voltage losses which increase with increasing cable length. The signal quality is also reduced with increasing cable length. A freely moving ROV (without umbilical cable) will be hampered by slow communication and with a signal quality which decreases with increasing operation depth.
The solution to the above mentioned problems is a riser tube tower comprising vertical wire connections between a platform, preferably a tension leg platform at the sea surface and a seabed frame comprising one or more wellheads in deep water, with guide plates arranged at least in one level, with spaced apertures, through which apertures pass riser tubes and wires. The new and inventive concept of this invention is characterized by:
shuttle plates arranged in the vertical spaces between the guide plates, where the shuttle plates are provided with spaced apertures, through which apertures pass riser tubes and wires, and
elevator devices arranged for vertical movements of the shuttle plates over, under, and between the guide plates, in such a way as to be vertically displaced together with the riser tubes to guide the riser tubes to the apertures of the guide plates, both between the platform and the seabed frame, and to the wellheads of the seabed frame.
The shuttle plates themselves may function as guide plates if the guide plates are not needed due to low current velocities or due to other circumstances. Thus the invention also comprises a riser tube tower comprising vertical wire connections between a platform, preferably a tension leg platform at the sea surface and a seabed frame in deep water having one or more wellheads, where the new and characterizing traits are:
a) at least one shuttle plate with spaced apertures through which apertures pass riser tubes and wires, and
b) elevator devices arranged for vertical movements of the shuttle plates, in such a way that the shuttle plates are arranged to be vertically displaced together with a riser tube to guide the riser tube""s lower end to the wellhead connectors of the seabed frame.
The invention comprises also a method for setting of riser tubes by means of vertical wire connections between a platform at the sea surface and a seabed frame, for example, a wellhead in deep water, by means of a device as described above, where the new and characterizing steps are:
i) elevating, by means of elevator devices, a shuttle plate to its upper position,
ii) lowering of the riser tube""s lower leading end through an adjacent aperture of the shuttle plate,
iii) lowering by means of the elevator devices the shuttle plate simultaneously with feeding out the riser tube""s lower leading end, until the lower leading end of the riser tube reaches into an aperture of an underlying shuttle plate,
iv) repeating steps (i) to (iii) with an underlying shuttle plate until the lower leading end of the riser tube reaches the planned depth level, preferably a wellhead of the seabed frame. Other and supplementary traits of the invention are defined in the patent claims.